1. Field of the Invention
This invention relates to a hydrogen-absorbing alloy for alkaline storage battery, and to an alkaline storage battery employing a positive electrode, a negative electrodes using the hydrogen-absorbing alloy for alkaline storage battery, and an alkaline electrolyte. More particularly, a feature of the invention is an improvement in a hydrogen-absorbing alloy for alkaline storage battery used for a negative electrode in an alkaline storage battery, which utilizes, for the purpose of enhancing hydrogen-absorbing capability of the hydrogen-absorbing alloy and improving a cycle life of the alkaline storage battery.
2. Description of Related Art
Conventionally, nickel-cadmium storage batteries have been commonly used as alkaline storage batteries. In recent years, nickel metal hydride batteries using a hydrogen-absorbing alloy as a material for their negative electrodes have drawn considerable attention from the viewpoints that they have higher capacity than nickel-cadmium storage batteries and, being free of cadmium, they are more environmentally safe.
As the nickel metal hydride batteries have been used in various portable devices, demands for further higher performance in the nickel metal hydride batteries have been increasing.
In the nickel metal hydride batteries, hydrogen-absorbing alloys such as a rare earth-nickel hydrogen-absorbing alloy having a CaCu5 crystal structure as its main phase and a Laves phase hydrogen-absorbing alloy containing Ti, Zr, V and Ni have been generally used for their negative electrodes.
However, these hydrogen-absorbing alloys generally do not necessarily have sufficient hydrogen-absorbing capability, and it has been difficult to increase the capacity of the alkaline storage batteries further.
In recent years, in order to improve the hydrogen-absorbing capability of the rare earth-nickel hydrogen-absorbing alloy, it has been proposed to use a hydrogen-absorbing alloy having a crystal structure other than CaCu5 type, by adding magnesium to the rare earth-nickel hydrogen-absorbing alloy (see Japanese Published Unexamined Patent Application No. 2002-69554).
Nevertheless, problems in producing the above-described hydrogen-absorbing alloy containing rare earth elements, nickel and magnesium by a metal mold casting process which is a conventional commonly used method, have been that it is difficult to obtain a hydrogen-absorbing alloy having an uniform structure, due to differences in melting point and specific gravity between magnesium and the other elements, and characteristics of the hydrogen-absorbing alloy such as corrosion resistance and a cycle life of the alkaline storage battery are degraded.
In this connection, there has been proposed a rapid quenching method to produce a hydrogen-absorbing alloy containing rare earth elements, nickel and magnesium with an uniform structure (see Japanese Published Unexamined Patent Application No. 2004-115870).
However, in producing the hydrogen-absorbing alloy containing the rare earth elements, nickel and magnesium by the rapid quenching method, there have still been the following problems. If a composition of the hydrogen-absorbing alloy is not suitable, the cycle life of the alkaline storage battery is degraded and hydrogen-absorbing capability of the hydrogen-absorbing alloy is lowered.